The present invention relates to a fuel cell separator, and a method for manufacturing a fuel cell separator.
Solid polymer fuel cells each include a membrane electrode assembly (MEA) including an electrolyte membrane composed of an ion exchange membrane and a pair of electrodes having the electrolyte membrane in between, and a pair of separators having the MEA in between. The pair of separators and the MEA define gas flow paths. Fuel gas (such as hydrogen gas) is supplied to one of the gas flow paths and oxidizing gas (such as air) is supplied to the other gas flow path. For the separator, metallic separators have been proposed to date. The metallic separators need to have high conductivity, high fuel gas tightness, and high corrosion resistance in oxidation reduction reactions of the fuel gas and oxidizing gas. Examples of materials for such metallic separators include stainless steel (SUS) and titanium.
Such metallic separators are known in Japanese Laid-Open Patent Publication No. 2004-14272 and Japanese Laid-Open Patent Publication No. 2006-269090. As shown in FIG. 21A, the separator according to Japanese Laid-Open Patent Publication No. 2004-14272 includes a metal substrate 100, and a conductive resin layer 106 on a surface of the metal substrate 100, the conductive resin layer 106 containing a mixture of a resin 102 and a conductive filler 104 of a metal carbide. The conductive resin layer 106 is configured such that the content of the conductive filler 104 per volume is continuously reduced from the metal substrate 100 (interface of the substrate) to the surface of the separator (hereinafter referred to as a first conventional separator).
Japanese Laid-Open Patent Publication No. 2004-14272 also proposes a metal separator (hereinafter referred to as a second conventional separator) as shown in FIG. 21B in which a low electric resistance layer 108 is provided on the surface of the conductive resin layer 106, the low electric resistance layer 108 containing a conductive filler 110 composed of a carbon material and having a volume resistivity lower than that of the surface of the conductive resin layer 106. The configuration shown in FIG. 21B brings an advantage of significantly reducing the contact resistance between the separator and a gas diffusion layer (GDL).
As shown in FIG. 23, Japanese Laid-Open Patent Publication No. 2006-269090 proposes a metal separator including a metal substrate 100, a first resin layer 120 on the surface of the metal substrate 100, and a second resin layer 130 on the surface of the first resin layer 120 (hereinafter referred to as a third conventional separator). The first resin layer 120 and the second resin layer 130 each contain a conductive filler 118. The volume resistivity of the second resin layer 130 is smaller than that volume resistivity of the first resin layer 120.
FIG. 22 shows a relationship between the position in the direction of thickness of the conductive layer or the resin layer and the proportion of the conductive filler (conductive particles) in the conventional examples. The solid line indicates the example shown in FIG. 21A, the dotted line indicates the example shown in FIG. 21B, and the long dashed short dashed line indicates the example in FIG. 23.
Japanese Laid-Open Patent Publication No. 2005-243354 proposes a method for manufacturing a fuel cell separator (referred to as a first conventional method). In the first conventional method, a first resin sheet 200 containing a conductive substance 202 and a second resin sheet 210 containing a conductive substance 202 are layered on a metal substrate 100 as shown in FIG. 24A to be bonded, and are integrally formed with the metal substrate 100 by heat press as shown in FIG. 24B. The content of the conductive substance 202 in the second resin sheet 210 is higher than the content of the conductive substance 202 in the first resin sheet 200. After integral formation, the separator is surface treated by heating at a temperature of the melting point of the resin or higher as shown in FIG. 24C.
In another typical method for manufacturing a fuel cell separator, as shown in FIG. 25, after a mixed solution of a conductive substance 140 and a resin is applied onto a metal substrate 100, and is cured to form a resin layer 152, a mixed solution of the conductive substance 140 and a resin is applied onto the surface of the resin layer 152, and is cured in the same manner. Thus, resin layers 154, 156, and 158 are sequentially formed (referred to as a second conventional method).
Japanese Laid-Open Patent Publication No. 2007-273458 discloses a method for manufacturing a fuel cell separator in which a mixed solution of a conductive ceramic and a resin (binder resin) is applied onto a metal substrate, and is dried in an oven to be burned.